In the past, direct converters, such as matrix converters, had a rather academic significance. However, nowadays, direct converters are gaining importance primarily in industrial applications since an input voltage or an input current with a first amplitude and a first frequency can be converted by means of a direct converter directly into an output voltage or into an output current with a second amplitude and a second frequency without a complicated DC voltage intermediate circuit or a DC intermediate circuit. A direct converter of this kind is specified, for example, in U.S. Pat. No. 6,900,998 B2. In U.S. Pat. No. 6,900,998 B2, the direct converter has n=3 input phase connections and p=3 output phase connections. Thus, the direct converter from U.S. Pat. No. 6,900,998 B2 is of three-phase design at the input end and at the output end. The direct converter from U.S. Pat. No. 6,900,998 B2 also includes nine phase modules having, in each case, a two-pole switching cell for switching a positive and a negative voltage between the poles, with each output phase connection being connected in series to each input phase connection in each case directly by means of a switching cell. A switching cell of this kind has drivable bidirectional power semiconductor switches with a controlled unidirectional current guidance direction and a capacitive energy storage means. The phase modules of the direct converter according to U.S. Pat. No. 6,900,998 B2 contain only switching cells and no inductances. Therefore, the voltage on each branch, that is to say on each two-pole switching cell, cannot be adjusted in such a way that a continuous current flow can be achieved from an input phase connection to an output phase connection, as a result of which it is not possible to actively set the current through the respective branch and through the respective phase module. The currents of the output phase connections of the phase modules are discretely switched to the individual phase modules, that is to say the currents of the individual phase modules describe discontinuous functions which jump in sections between zero and the instantaneous value of the current in the associated output phase connection. The connection and disconnection of the currents to the output phase connections of the phase modules requires a complex control algorithm which is solved by means of complicated tables. Care should be taken here that individual phases must never be shorted to form a voltage mesh since the total voltages of two phase modules are normally not exactly identical. At the same time, the current flow in the connections must never be interrupted, that is to say the phase modules always have to be switched at exactly the same time, which requires a considerable amount of expenditure on control. In addition, the connection and disconnection of the full currents leads to a high effective current (RMS current) with correspondingly high losses in the components and, in particular, in the power semiconductor switches.
The power of a phase module and, in particular, also the power of all switching cells of a phase module can deviate on average from zero for a variety of reasons if the phase module has, for example, a plurality of switching cells. Possible causes can be insufficient control, transient operating states, and specific symmetries between the switching operations and the currents through the phase modules, or else physical differences between individual components. If the mean voltage deviation of the capacitive energy storage means of all the switching cells of a phase module is not equal to zero, the result may be a considerable deviation in the voltage across the phase module, which in turn leads to uncontrolled circulating currents through the phase modules.
Furthermore, no, or only very limited, exchange of electrical energy between individual branches is possible with the direct converter from U.S. Pat. No. 6,900,998 B2. However, if the direct converter were to be in a position to transmit a large quantity of electrical energy, the capacitances of the switching cells of U.S. Pat. No. 6,900,998 B2 have to be of correspondingly large dimensions, which results in an enormous amount of space being required by a direct converter of this kind and in considerable costs. Systems which are designed with direct converters of this kind will therefore likewise have a correspondingly large space requirement and be correspondingly expensive.
WO 2008/067788 A1 discloses a method for operating the converter circuit according to WO 2007/023064 A1 which controls the energy content of the switching cells. The method described in WO 2008/067788 A1 applies only to designs of the converter circuit according to WO 2007/023064 A1 which connect three phases of one system to two phases of another system. “On Dynamics and Voltage Control of the Modular Multilevel Converter”, Power Electronics and Applications, 2009, EPE 2009, 13th European Conference on IEEE, Sep. 18, 2009 discloses a further method for operating a converter circuit, in which each phase module has a plurality of two-pole switching cells which are connected to one another in series, and each switching cell has drivable bidirectional power semiconductor switches with a controlled unidirectional current guidance direction and a capacitive energy storage means. Furthermore, a method of this generic type for operating a converter circuit is also disclosed in WO 2008/067784 A1.